fix: use @yarnpkg/cli instead of yarn@berry/1.x

yarn@berry and yarn@latest install yarn 1.x, not yarn 4.x.
@yarnpkg/cli is the actual yarn 4.x npm package. Also restore
yarn 1.x before 'yarn set version stable' on METHOD 4.

npm_install.py

@@ -2232,35 +2232,54 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
                     if DEBUG:
                         print(f"   ⚠ 'yarn set version' failed: {e}")
             
-            # METHOD 3: npm install yarn@berry
+            # METHOD 3: npm install @yarnpkg/cli (yarn 4.x)
             try:
-                run(["npm", "install", "-g", "yarn@berry"], check=False)
+                run(["npm", "install", "-g", "@yarnpkg/cli"], check=False)
                 
                 if shutil.which("yarn"):
                     major = _get_yarn_major_version(["yarn"])
                     
                     if major and major >= 4:
                         if DEBUG:
-                            print(f"   ✓ Yarn {major}.x installed via npm")
+                            print(f"   ✓ Yarn {major}.x installed via @yarnpkg/cli")
                         return True
             except Exception as e:
                 if DEBUG:
-                    print(f"   ⚠ npm install yarn@berry failed: {e}")
+                    print(f"   ⚠ npm install @yarnpkg/cli failed: {e}")
             
-            # METHOD 4: Force install latest yarn
+            # METHOD 4: yarn set version stable (requires yarn 1.x)
+            # First install yarn 1.x if not present
+            if not shutil.which("yarn"):
                 try:
-                run(["npm", "install", "-g", "yarn@latest", "--force"], check=False)
+                    run(["npm", "install", "-g", "yarn@1"], check=False)
+                except Exception:
+                    pass
             
             if shutil.which("yarn"):
+                try:
+                    # Uninstall any global yarn 4.x first
+                    run(["npm", "uninstall", "-g", "@yarnpkg/cli"], check=False,
+                        stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+                    
+                    # yarn 1.x can upgrade itself to 4.x
+                    result = subprocess.run(
+                        ["yarn", "set", "version", "stable"],
+                        capture_output=True,
+                        timeout=120,
+                        text=True,
+                        check=False,
+                    )
+                    
+                    if result.returncode == 0:
                         major = _get_yarn_major_version(["yarn"])
                         
                         if major and major >= 4:
                             if DEBUG:
-                            print(f"   ✓ Yarn {major}.x installed via npm --force")
+                                print(f"   ✓ Yarn {major}.x installed via 'yarn set version'")
                             return True
                 except Exception as e:
                     if DEBUG:
-                    print(f"   ⚠ npm install --force failed: {e}")
+                        print(f"   ⚠ 'yarn set version' failed: {e}")
             
             return False